1. Field of the Invention
The present invention relates to a sheet-through type document reader for use in an image forming apparatus such as a digital copying machine or a multi-functional apparatus having a copying function and a facsimile function. More particularly, the invention relates to a sheet-through type document reader which is advantageous for preventing occurrence of a black line in a reproduced image which is attributed to dust, dirt, a scratch or the like present on an upper or lower surface of a contact glass.
2. Description of Related Art
Digital image forming apparatuses represented by digital copying machines and multi-functional image forming apparatuses are typically provided with a sheet-through type document reader. The sheet-through type document reader is adapted to read an image of a document original during passage of the document original through a stationary reading section thereof. Where a copy is made from a document original or image data of a document original is taken into a computer by means of such a sheet-through type document reader, it is important to reproduce an original image with a high fidelity.
In reality, however, an abnormal image portion which is different from the corresponding portion of the original image is often observed in a reproduced image (an image of the copy or an image reproduced on the basis of the image data). The abnormal image is produced due to malfunction of a reading system, an image forming system or an image processing system, and a variety of image abnormalities could occur depending on which system malfunctions and what kind of trouble the system suffers from.
Particularly, local image abnormalities are conspicuous and, therefore, consideration should be given to the local image abnormalities. Examples of the local image abnormalities include black spots and black lines formed on a white background portion and omissions such as white spots and white lines formed on a solid image portion. In general, these image abnormalities rarely occur when the sheet-through document reader is brand-new, but progressively occur during prolonged use.
A major cause of the image abnormalities is that dust in air and paper dust from document originals transported through the reader adhere on optical components of a reader optical system and contaminate the optical components. More specifically, contamination of an upper or lower surface of a contact glass or contamination of a mirror, a lens or the like is a common cause.
The problem of the contamination of the optical components can be somewhat alleviated by completely sealing a housing space of the reader optical system. However, a document illumination lamp as a heat source coexists with an image sensor (CCD) for sensing an optical image of a document original, so that cooling means is required for cooling the image sensor which is highly temperature-dependent. Therefore, it is virtually impossible to completely seal the housing space of the reader optical system.
In the sheet-through type document reader, a dust filter is provided in an air inlet of an enclosure for protection against dust. However, the aforesaid image abnormalities may also occur due to minute dust particles having passed through the dust filter and, therefore, cannot completely be prevented by the provision of the dust filter alone.
A current approach to this problem is that a user or a service man takes proper countermeasures when an image abnormality occurs. More specifically, the user cleans the upper surface of the contact glass for decontamination. The service man cleans the lower surface of the contact glass, the mirrors and the lenses for decontamination at maintenance.
However, when an image abnormality occurs, troubleshooting is difficult even for a skilled service man who has enough knowledge about the sheet-through type document reader, and is time-consuming.
In the case of a digital copying machine, for example, which of a printer section, a reading section and an image processing section the cause of an image abnormality resides in should be inferred from the kind of the image abnormality and the like. Even if it has been found that the cause resides in the reading section, it should further be determined which of optical components of the reading section the cause resides in. Thus, the troubleshooting requires a complicated and troublesome operation.
It is an object of the present invention to provide a sheet-through type document reader which effectively prevents image abnormalities due to the dust or the like presented on a contact glass, thereby making the troubleshooting easier.
In accordance with one aspect of the present invention, there is provided a sheet-through type document reader for reading an image formed on a surface of a document original being transported through a reading position, the document reader comprising: a reading mechanism for performing main scanning to scan the surface of the document original in a direction perpendicular to a document transport direction at the reading position thereby to read an optical image of the document original, and converting the optical image into image signals; an abnormal value detection circuit for determining whether or not any abnormal value is present in image signals obtained by performing the main scanning in predetermined timing by means of the reading mechanism; and a reading position shifting mechanism for shifting the reading position in the document transport direction when the abnormal value detection circuit detects an abnormal value.
The abnormal value detection circuit is preferably adapted to determine whether or not any abnormal value is present in image signals obtained by performing the main scanning by means of the reading mechanism, for example, before the document original to be read reaches the reading position.
In accordance with the present invention, dust, dirt or a scratch present on an upper or lower surface of a contact surface is detected by detecting the abnormal value in the image signals. In response to the detection of the abnormal value, the reading position is shifted in the document transport direction. Thus, occurrence of an image abnormality such as a black line can be prevented. That is, the document reader of the present invention is adapted to detect the dust, dirt or scratch present on the upper or lower surface of the contact glass and perform a document reading operation at a reading position apart from the position where the dust or the like has been detected.
Thus, the image abnormality due to the dust or the like present on the upper or lower surface of the contact glass can be avoided by the automatic shift of the reading position in accordance with the present invention. Therefore, where an image abnormality occurs in a copying machine or the like provided with this document reader, it is merely necessary to check components other than the contact glass, thereby facilitating the troubleshooting.
In accordance with another aspect of the invention, there is provided a document reading process which comprises the steps of: transporting a document original through a reading position; performing a document reading operation by scanning a surface of the document original in a direction perpendicular to a document transport direction at the reading position for main scanning, and converting an optical image of the document original into image signals by means of a reading mechanism; performing an abnormal value detecting operation by determining whether or not any abnormal value is present in image signals obtained by performing the main scanning in predetermined timing by means of the reading mechanism; and shifting the reading position in the document transport direction when an abnormal value is detected in the image signals.